Back electrodes for solar cells and the like are known in the art. For example and without limitation, see U.S. Patent Document Nos. 2004/0261841, 2005/0253142, U.S. Pat. No. 5,981,868, 2005/0236032, U.S. Pat. No. 5,477,088, 2005/0186342, 2005/0257825, 2005/0284518, 2004/0087172, and 2005/0072461, the disclosures of which are hereby incorporated herein by reference.
Back electrodes for electronic devices such as CIS (copper indium diselenide) and CIGS (copper indium gallium diselenide) based solar cells are typically made of a layer of Mo which is sputter-deposited directly onto a substrate. In certain applications, the substrate may be made of glass such as soda-lime-silica based glass. After a series of following depositions and scribing/patterning steps, the device is completed by bus bar contacts, encapsulation and framing. Each component of the device contributes to efficiency.
With respect to glass substrates, soda-lime-silica based float glass contains Na (e.g., in the form of Na2O). It is known that sometimes one desires to provide Na in a polycrystalline CIGS absorber layer so as to increase solar cell efficiencies. Accordingly, unlike many other electo-optical coatings, CIS and CIGS are typically fairly sodium friendly. However, if one decides to introduce Na into a CIS and/or CIGS absorbing layer, it is important that the sodium be introduced in a controllable and predictable manner.
In making solar cells and other electronic devices, semiconductor layers are often subjected to significant heat treating during the manufacturing process. Such heat treating may include subjecting the glass substrate and the semiconductor layer(s) to temperatures of at least 400 degrees C., sometimes at least 500 degrees C., and possibly higher temperatures. Unfortunately, such heat treating temperatures often cause diffusion of Na from the glass substrate into the coating in an uncontrollable and predictable manner. This means that uncontrolled and unpredictable amounts of sodium can diffuse from the glass substrate into a CIGS or CIS layer in a solar cell or other electronic device, which can lead to undesirable and/or unpredictable functionality in certain instances.
In view of the above, it will be apparent that there exists a need in the art for a technique to prevent or reduce uncontrolled sodium migration into semiconductor layer(s) of solar cells or other electronic devices.